Less cognitive decline in older adults who get average amount of sleep, study says

Short and long sleepers suffer greater cognitive decline than those who get an average amount of sleep even when the effects of Alzheimer’s disease are taken into account, according to a new multi-year study of older adults.

A team of researchers at Washington University School of Medicine in St Louis tracked the cognitive functions in a large group of older adults over several years and analysed this against levels of Alzheimer’s-related proteins and measures of brain activity during sleep.

The researchers found a ‘U-shaped’ relationship between sleep and cognitive decline. Overall, cognitive scores declined for the groups that slept less than 4.5 (short sleepers) or more than 6.5 hours per night (long sleepers) while scores stayed stable for those in the middle of the range.

“Our study suggests that there is a middle range, or ‘sweet spot,’ for total sleep time where cognitive performance was stable over time,” Brendan Lucey, MD, an associate professor of neurology and director of the Washington University Sleep Medicine Center and first author of the study, said.

He added: “Short and long sleep times were associated with worse cognitive performance, perhaps due to insufficient sleep or poor sleep quality. An unanswered question is if we can intervene to improve sleep, such as increasing sleep time for short sleepers by an hour or so, would that have a positive effect on their cognitive performance so they no longer decline? We need more longitudinal data to answer this question.”

The researchers said their findings could aid efforts to help keep people’s minds sharp as they age.

Previous studies have shown that self-reported short and long sleepers are both more likely to perform poorly on cognitive tests, but such studies typically do not include assessments of Alzheimer’s disease, the researchers said.

Poor sleep and Alzheimer’s disease are both associated with cognitive decline. Alzheimer’s is the main cause of cognitive decline in older adults, contributing to about 70 per cent of dementia cases. Poor sleep is a common symptom of the disease and a driving force that can accelerate the disease’s progression.

For their study, published in the Brain journal, the researchers turned to volunteers who participate in Alzheimer’s studies through the university’s Charles F and Joanne Knight Alzheimer Disease Research Center.

Such volunteers undergo annual clinical and cognitive assessments, and provide a blood sample to be tested for the high-risk Alzheimer’s genetic variant APOE4.

For this study, the participants also provided samples of cerebrospinal fluid to measure levels of Alzheimer’s proteins, and each slept with a tiny electroencephalogram (EEG) monitor strapped to their foreheads for four to six nights to measure brain activity during sleep.

In total, the researchers obtained sleep and Alzheimer’s data on 100 participants whose cognitive function had been monitored for an average of 4.5years. Most (88) had no cognitive impairments, 11 were very mildly impaired, and one had mild cognitive impairment. The average age was 75 at the time of the sleep study.

Overall, cognitive scores declined for the groups that slept less than 4.5 or more than 6.5 hours per night — as measured by EEG — while scores stayed stable for those in the middle of the range.

EEG tends to yield estimates of sleep time that are about an hour shorter than self-reported sleep time, so the findings correspond to 5.5 to 7.5 hours of self-reported sleep, Prof Lucey said.

The U-shaped relationship held true for measures of specific sleep phases, the researchers said, including rapid-eye movement (REM), or dreaming, sleep; and non-REM sleep. Moreover, the relationship held even after adjusting for factors that can affect both sleep and cognition, such as age, sex, levels of Alzheimer’s proteins, and the presence of APOE4, the paper said.

“It was particularly interesting to see that not only those with short amounts of sleep but also those with long amounts of sleep had more cognitive decline,” David Holtzman, MD, a professor of neurology and co-senior author, said.

“It suggests that sleep quality may be key, as opposed to simply total sleep.”